Modern society has quickly adopted, and become reliant upon, handheld devices for wireless communication. For example, cellular telephones continue to proliferate in the global marketplace due to technological improvements in both the communication quality and device functionality. These wireless communication devices have become common for both personal and business use, allowing users to transmit and receive voice, text and graphical data from a multitude of geographic locations. The communication networks utilized by these devices span different frequencies and cover different transmission distances, each having strengths desirable for various applications.
Cellular networks facilitate wireless communication over large geographic areas. These network technologies have commonly been divided by generations, starting in the late 1970s to early 1980s with first generation (1G) analog cellular telephones that provided baseline voice communication, to modern digital cellular telephones. Global System for Mobile Communications (GSM) is an example of a widely employed 2G digital cellular network communicating in the 900 MHz/1.8 GHz bands in Europe and at 850 MHz and 1.9 GHz in the United States. This network provides voice communication and also supports the transmission of textual data via the Short Messaging Service (SMS). SMS allows a wireless communications device (WCD) to transmit and receive text messages of up to 160 characters, while providing data transfer to packet networks, Integrated Services Digital Network (ISDN) and Plain Old Telephone Service (POTS) users at 9.6 Kbps. The Multimedia Messaging Service (MMS), an enhanced messaging system allowing for the transmission of sound, graphics and video files in addition to simple text, has also become available in certain devices. Soon, emerging technologies such as Digital Video Broadcasting for Handheld Devices (DVB-H) will make streaming digital video, and other similar content, available via direct transmission to a WCD. While long-range communication networks like GSM are a well-accepted means for transmitting and receiving data, due to cost, traffic and legislative concerns, these networks may not be appropriate for all data applications.
Short-range wireless networks provide communication solutions that avoid some of the problems seen in large cellular networks. Bluetooth™ is an example of a short-range wireless technology quickly gaining acceptance in the marketplace. In addition to Bluetooth™, other popular short-range wireless networks include for example IEEE 802.11 Wireless LAN, Wireless Universal Serial Bus (WUSB), Ultra Wideband (UWB), ZigBee (IEEE 802.15.4 and IEEE 802.15.4a), wherein each of these exemplary wireless mediums have features and advantages that make them appropriate for various applications The IEEE 802.11 Wireless LAN is a popular short-range wireless network. The IEEE 802.11 Wireless LAN Standard defines a medium access control (MAC) specification and includes several physical layers (PHY) that specify the over-the-air modulation techniques that all use the same basic MAC protocol. The OFDM PHY for the 5 GHz band (formerly known as the 802.11a standard) uses orthogonal frequency-division multiplexing (OFDM) with a maximum data rate of 54 Mbit/s. The DSSS PHY for the 2.4 GHz band (formerly known as the 802.11b standard) uses direct sequence spread spectrum (DSSS) modulation to deliver up to 11 Mbps data rates. The ERP PHY (formerly known as the 802.11g standard) uses the 2.4 GHz band, and builds on top of the DSSS PHY providing data rates up to 54 Mbps with OFDM based modes similar to the ones in the OFDM PHY for the 5 GHz band. The radiation pattern for devices using these PHYs is omnidirectional, wherein power is radiated uniformly in a plane.
The IEEE 802.11 Wireless LAN Standards describe two major components, a wireless device, called a station (STA), and an access point (AP) wireless device. The AP may perform the wireless-to-wired bridging from STAs to a wired network. The basic network is the basic service set (BSS), which is a group of wireless devices that communicate with each other. An infrastructure BSS is a network that has an AP as an essential node.
IEEE 802.11 medium access control (MAC) protocol regulates access to the RF physical link. The MAC provides a basic access mechanism with clear channel assessment, channel synchronization, and collision avoidance using the Carrier sense Multiple Access (CSMA) principle. It also provides network inquiring, which is an inquiry and scan operation. The MAC provides data fragmentation, authentication, encryption, and power management.
Synchronization is the process of the stations in an IEEE 802.11 network getting in step with each other, so that reliable communication is possible. The MAC provides the synchronization mechanism to allow support of physical layers that make use of frequency hopping or other time-based mechanisms where the parameters of the physical layer change with time. The process involves beaconing to announce the presence of a network and inquiring to find a network. Once a network is found, a station joins the network.
An IEEE 802.11 ad hoc network is referred to as an independent BSS (IBSS). In an IEEE 802.11 ad hoc network, there is no access point (AP) to act as the central time source for the ad hoc network. The IBSS is the most basic type of IEEE 802.11 LAN, a minimum IEEE 802.11 LAN may consist of only two STAs. The BSSID field of a MAC uniquely identifies each BSS. The value of this field in an IBSS is a random number used to provide a high probability of selecting a unique BSSID. In addition, the service set identifier (SSID) indicates the identity of an IBSS, as a network ID unique to a network. Only stations that share the same SSID and BSSID are able to communicate with each other.
Since there is no AP, the mobile station that starts the ad hoc network will begin by transmitting a Beacon, selecting a unique BSSID and choosing a beacon period. This establishes the basic beaconing process for this ad hoc network. After the ad hoc network has been established, each station in the ad hoc network will attempt to send a Beacon after the target beacon transmission time arrives. To minimize actual collisions of the transmitted Beacon frames on the medium, each station in the ad hoc network will choose a random delay value, which it will allow to expire before it attempts its Beacon transmission. If the station receives a beacon from another station in the network when waiting for the delay to expire, it will not transmit its own beacon.
In order for a station to communicate with other stations in a wireless network, it must first find the other stations. The process of finding another station may involve either passive scanning or active scanning. Passive scanning involves only listening for example for IEEE 802.11 traffic. Active scanning requires the inquiring station to transmit and invoke responses from IEEE 802.11 stations with probe request frames.